This invention relates generally to electrical control circuitry for the prevention of undesired reapplication of power to a load device in event of a power outage and more particularly to apparatus for controlling the application of power to a load device such as a shunt field DC motor which has rectified AC power supplied thereto from a phase-controlled circuit including controlled semiconductor switch means such as silicon control rectifiers.
Solid state electronic controls for motor speed control of a direct current shunt field motor are well known to those skilled in the art. Such a teaching is contained in U.S. Pat. No. 3,555,386, "Control Apparatus for Motors and the Like," F. O. Wisman and the above referenced related application, U.S. Ser. No. 605,621, now U.S. Pat. No. 4,010,411. Such systems utilize AC to DC conversion apparatus which is adapted to provide a controlled application of rectified AC power to the motor's armature circuit by means of the synchronous cyclic conduction of silicon controlled rectifiers (SCR). The SCR's are driven into conduction by means of trigger pulses applied from a pulse generator which receives its DC power supply potential from a bridge circuit also coupled to the AC power.
In some usages of such a circuit arrangement, it may be undesirable or hazardous if power is restored automatically to the motor in the event of an AC power outage which is then subsequently restored. It has long been known to preclude such unpredictable starting of motors and other types of machinery by incorporating circuit interrupters in the high power (AC) side of the line, for example magnetic contactors initially energized by momentary manual push buttons and held closed by one of their own contacts or by the use of a latching relay. However, the contactor approach is undesirable, particularly in lower power applications, for reasons of reliability, bulk, weight, cost and vulnerability to hostile environmental influences such as vibration and dust.